The present invention is generically directed on a technique for leak testing closed and filled containers, whereby the filling material comprises at least one liquid component.
Leak testing techniques according to which closed containers are introduced in a test cavity which, after having sealingly been closed, is lowered in pressure by a suctioning pump are known. If the container is not leaking, then once a predetermined pressure has been reached in the test cavity and thus in the surrounding of a container to be tested; this pressure will be kept substantially constant. If a leak is provided in an area of the container, wherein air is entrapped, a flow of air out of the container will lead to a rise of the surrounding pressure. If a leak is present in the area of the container where filling good is entrapped, the question whether such leak will lead to a significant rise of the surrounding pressure is largely dependent on the kind of filling good as of its viscosity, whether solid particles are present in the filling good and, obviously, on the largeness of the leak.
Different approaches have become known to accurately detect leaks at such product-filled containers, irrespective whether the leak is present in an air entrapping container area or in a container area covered with filling good. One such approach which is the topic of the co-pending European patent application EP-A-0 791 814 and the U.S. patent application Ser. No. 08/862,993 proposes to provide an impedance measurement, specifically a resistance measurement, just adjacent to the outer wall of the container by means of an electrode arrangement: As soon as liquid emerges from a leak it will contact a respective pair of impedance measuring electrodes and lead to a significant change of impedance measured between such electrodes.
Nevertheless, such an approach necessitates considerable additional expenditure with respect to provision of the impedance measuring arrangement in each test cavity, especially of a multi-cavity in-line inspection machine and does not enable detection of very small leaks far below of one micron and largely independent from container shape and kind of filling good.
It is a primary object of the present invention to provide a leakage test method and apparatus, which may be applied to a very large scale of different containers and of different filling goods, provided at least one component thereof being liquid.
It is a further object of the present invention to provide such method and apparatus which are rather inexpensive with respect to electronic and further equipment, and which thus allow for very economic testing.
It is still further an object of the present invention to provide such method and apparatus which have a short measuring cycle and nevertheless a very high measuring accuracy.
These objects are realised by the testing method for leak testing at least one closed and filled container, whereby the content of the container comprises at least one liquid component and wherein a pressure difference is applied across at least a part of the wall of the container which part is to be leak tested and wherein the applied pressure difference is directed towards the surrounding of the container and wherein further the pressure in the surrounding of the container is monitored as a leak indicative signal which is characterised by the fact that the pressure difference is established by lowering the pressure in the surrounding of the container at least to a value which accords to the vapour pressure of the at least one liquid component of the filling product of the container to be tested.
The present invention departs from the recognition that if a container is leaking and liquid is drawn by the lower surrounding pressure to its outside this willxe2x80x94at a constant volume of the surroundingxe2x80x94lead to evaporation of the liquid as soon as the surrounding pressure reaches its vapour pressure. This leads to a significant change in surrounding pressure compared with the surrounding pressure which would establish at the same measuring conditions but with an unleaking container.
Monitoring the pressure in a test cavity containing the container, once vapour pressure of the possibly leaking liquid is reached reveals as being a very accurate technique for leak testing. It has been noted that by such a technique leak detection of containers with a very large spectrum of filling products may accurately be performed and that leaks at present moment down to 0.02 xcexcm are accurately detectable.
Further, it has been noted that the volume of the test cavity is uncritical, so that by the inventive technique it becomes possible to simultaneously test batches of containers, thereby accurately detecting if one container of such a container batch is leaking.
As soon as the pressure surrounding a leaking container is lowered with respect to its interior pressure, some of the liquid is suctioned out of the container and as soon as the surrounding pressure reaches vapour pressure it starts to evaporate. As at a constant volume of the surrounding area of the container evaporation of the liquid leads to increase of pressure and the pump lowering the surrounding pressure must now remove vapour of the liquid too, significant measurements may be done especially after the surrounding pressure of the container becomes lower than the said vapour pressure. Nevertheless, it is preferred to provide pumping abilities which may evacuate the surrounding of the container to be tested to a significantly lower value than said vapour pressure, namely by at least two, preferably even by at least three decades.
As a leak-significant pressure change may be detected as soon as one of possibly several liquid components of the filling good starts to evaporatexe2x80x94in the case the content of the container contains more than one liquid componentxe2x80x94it is recommended to select the vapour pressure of that component of the several liquid components which is the higher and to lower the pressure of the surrounding of the container at least to that vapour pressure value.
Although and as well known vapour pressure is a function of temperature and thus it might be advantageous in some cases e.g. to heat the surrounding of the container to a predetermined temperature so as to settle the relevant vapour pressure for a predetermined liquid, the inventive method and apparatus becomes significantly less complex if the test is performed at room temperature, and thus the vapour pressure to be at least reached is considered at room temperature, i.e. around 20xc2x0 C.
Further, a very accurate leak detection becomes possible if the surrounding pressure of the container is measured at two subsequent points in time, whereby we understand under xe2x80x9cpointxe2x80x9d that interval of time necessary for accurately measuring the prevailing pressure. Although it is absolutely possible to realise leak detection by applying the pumping action of the evacuating pump to the surrounding of the container and then by measuring the resulting surrounding absolute pressure after a predetermined time span, the said measuring of the surrounding pressure at two specific points in time allows to use the first value is measured as a reference value and then to form the difference of the second value measured with respect to the reference value. There is thereby realised a pressure difference measurement instead of an absolute pressure measurement. More specifically, the first pressure signal which is measured at the first point in time is stored as an electric signal, then, after having measured the second pressure value, a difference is formed between the first value (still stored) and the second value.
The PCT patent application No. WO94/05991 with its U.S. counter-part No. U.S. Pat. No. 5,239,859, assigned to the same applicant as the present invention, describes a method and apparatus for very accurately offset-compensated pressure difference measurement. In a preferred mode of operating the method according to the present invention as well as of realising the inventive apparatus, that pressure difference measuring technique and apparatus are used. Therefore, the WO94/05991 or the respective U.S. Pat. No. 5,239,859 are fully incorporated by reference in this present disclosure, although, and as will be seen most important features are specifically described also in this present application.
Because it is largely uncritical how big the surrounding volume of a test cavity for the container is, with respect to the volume of the container to be tested, the inventive method and apparatus reveals to have a further significant advantage:
If the wall of the at least one container to be tested withstands the pressure difference between container internal pressure (normally ambient pressure) and lowered surrounding pressure, such a container may simply be introduced in the test cavity forming the surrounding, largely irrespective how big such container is with respect to the test cavity. Nevertheless, a highly accurate indication of leakage will inventively be gained. Therefore, one and the same test cavity may be used for a large number of differently sized and different-volume containers. This results in a further advantage in that batches of more than one, even of a multitude of containers, may be introduced in one test cavity forming the surrounding and although one single container occupying only a small percentage of the overall cavity volume, an accurate leak indication will be detected if even only one of the batch-containers is leaking into the surrounding atmosphere.
A further significant advantage of the present invention is the following:
Sometimes the filled containers are not completely filled, but there is some amount of air entrapped in the closed container. If a leak is present in that area of such a container, which is adjacent to entrapped air or gas, by lowering the surrounding pressure, such air will be suctioned through the leak out of the container. With the pressure of the entrapped air in the container becoming progressively lower, there will also start vaporisation of the liquid component within the container and such vapour will also leave through the leak. Both, namely first the air leaving through the leak, then vapour leaving through the leak, will enlarge the surrounding pressure so that a leak in an entrapped air region of the container will lead to a change in the surrounding pressure, i.e. to rising of said pressure, as if the leak was in the liquid content covered area of the container wall. Thus, by properly setting a threshold value for leak detection according to the smallest still tolerated pressure change in the surrounding, it becomes uncritical whether such leak is present at an air-covered container area or at a content-covered container area.
If one and the same leak at an air-entrapped area of the container leads to a smaller pressure change in the surrounding, than the same leak would generate if situated at a liquid-covered container area, it is such a pressure change which will govern setting of a threshold value to detect whether a container is leaky or not. If, inversely, one and the same leak in a liquid-covered area would result in a smaller pressure change in the surrounding than such leak in an air-contacted wall area, then it is again that smaller pressure change which governs the threshold setting for detecting leaking/not leaking containers.
If a container under test is largely leaky, lowering of the surrounding pressure should be stopped as soon as such leaking is detected so as to prevent the content of the container to spoil the interior of the test cavity or, generally spoken, the surrounding of the container and possibly even the pumping arrangement more than absolutely necessary. This is realised either by monitoring whether the pumping action results in a predetermined lowering of surrounding pressure or not or one may detect spreading of content of the container into its surrounding by means of an impedance, thereby preferably a DC resistance measurement in the surrounding of the container just adjacent to the wall of the container which is to be tested. This is realised by providing an electrode arrangement in said adjacent surrounding and all around at least that part of the container to be tested. As soon as filling content of the container is suctioned to its outer wall, the electrode arrangement will be bridged by such content, leading abruptly to an indicative impedance change which, after having been detected, is used to stop further pressure lowering at the surrounding of the container.
This latter technique of rapidly detecting large leaks is applied especially to containers where it is necessary to snugly encapsulate them in the test cavity because their walls would not stand the pressure difference applied. In such a case the electrode arrangement for impedance measurement may be incorporated along the inner wall of the test cavity, which snugly fits with the at least one container. If such container is to be tested and therefore the test cavity snugly fits its shape, nevertheless a continuous volume is maintained between the outer wall of the container and the wall of the test cavity for defining the surrounding of the container by providing a sustaining grid or mesh inlay or preferably by roughening the interior wall of the test cavity so that a multitude of micro-embossments of the test cavity wall sustain the container wall and prevent it from further outward bowing due to the applied pressure difference. Thereby, the intercommunicating space between such embossments defines for the surrounding space of the container.
Once the container in a test cavity, defining for its surrounding, has been detected as being leaky, it is probable that such test cavity will be contaminated by some of the container""s content. Then, such cavity is cleaned after the leaky container has been removed, be it by evacuation and/or flushing with a flushing gas, preferably nitrogen, be it by heating or by combining these techniques, e.g. by a heated flushing gas.
If the inventive method or apparatus is applied for in-line testing containers and thus two or more of the inventive methods and of the respective apparatus are operated in parallel on a set of containers and one of such containers is detected to be leaky, then the respective test cavity defining for its surrounding is not anymore filled with a container at the next measuring cycle, but is kept empty, using that cycle during which the other cavities are in testing condition for cleaning and reconditioning the probably contaminated cavity. Further, it is proposed in some cases to accelerate squeezing-out of liquid, if a leak is present, by mechanically biasing the wall of the container inwardly, thus rising its interior pressure over atmospheric pressure.
To fulfill the object, the present invention proposes a leak testing apparatus for leak testing at least one closed and filled container, whereby the content of the container comprises at least one liquid component, which comprises at least one sealingly closable test cavity and at least one evacuation pump operationally connected to the test cavity and further at least one pressure sensor operationally connected to the test cavity, whereby the evacuation pump is selected so as to be able to pump the test cavity to at least vapour pressure of the liquid component of the container content, approx. at room temperature and the pressure sensor is a vacuum pressure sensor, preferably comprising at least a Pirani sensor stage.
Preferred embodiments of the inventive method and inventive apparatus are disclosed hereinafter. The inventive method and apparatus may preferably be used for leak testing blisters, vials, medical application containers, foodstuff or beverage containers, and tanks. Thereby, it must be pointed out that besides of leak testing of smaller containers, the present invention makes it possible to permanently monitor tightness of the tanks of huge tank plants, as for gasoline, gases, etc., e.g., on train or street transports, thereby generating an alarm signal as soon as a leak is detected.